The concurrent monitoring of oxygen consumption and anesthesia uptake during surgery provides significant information which can assist the attending physician in intraoperative and post operative patient care. In particular, whole body oxygen consumption (VO.sub.2) can indicate physiological changes which may prove fatal if not properly treated. Such conditions include respiratory failure, injury and many pathophysiological disorders resulting from inadequate cellular metabolism. VO.sub.2 in critically ill patients serves as a measure of cardiovascular function and tissue perfusion, as well as a parameter for the calculation of cardiac output by the Fick principle. Furthermore, oxygen transfer and transport requirements during surgery can be monitored in view of the dependence of such functions on the rate of VO.sub.2 by the tissues.
Concurrently with oxygen consumption measurements, the rate of uptake of anesthesia may require monitoring, particularly in a closed system or rebreathing administration apparatus. In addition to enabling maintenance of appropriate tissue saturation, a knowledge of actual anesthesia gas uptake may provide useful information as a research tool in evaluating anesthetics and various forms of instrumentation.
Numerous methods for measuring VO.sub.2 have been used, all but a few of which fit into one of two groups. The most often used and simplest method is the closed circuit method which uses a spirometer filled with oxygen from which the patient breathes. The reduction in volume gives direct measurement of VO.sub.2. The second method, the open circuit method, calculates VO.sub.2 by subtracting the product of the expired percent oxygen and expired volume from the product of the inspired percent and volume.
The need for monitoring VO.sub.2 of critically-ill and surgical patients who often require artificial ventilation with oxygen-enriched air is not filled by either of these methods. The closed circuit spirometer method does not provide a continuous record and is very susceptible to leaks or changes in lung volume, thereby creating a non-steady state condition with regard to nitrogen or the concurrently applied nitrous oxide. The open circuit method does provide a continuous record but its accuracy is decreased due to variations in oxygen concentration supplied to the patient. Since the oxygen concentration may change during inspiration and from breath to breath, flow weighted averages of both inspired and expired air are needed. During anesthesia, the open circuit technique requires the measurement of both inspired and expired volumes since the simple calculations of these volumes become inaccurate when dealing with such gases as nitrous oxide. Equipment for measuring these volumes is traditionally very bulky and requires a major revision of the conventional anesthesia delivery system. For these reasons, it is suggested that a closed system is preferred in monitoring oxygen consumption in surgical patients.
In cases where a closed anesthesia delivery system has been utilized, monitoring of oxygen and nitrous oxide has been a manual operation for the anesthesiologist. By observing a bellows system, anesthesia uptake is observed and resupply is effected by opening valves to release sufficient gas to bring the volume to an appropriate level. The oxygen concentration is maintained by similar means, relying perhaps on an oxygen sensor to indicate oxygen deficiency. The primary disadvantages of such a system include uncertainty as to actual consumption and uptake, inability to monitor rates of consumption and uptake, necessity to rely on subjective judgment of the anesthesiologist to make the appropriate replenishment, and the inconvenience and danger of requiring the constant attention of the anesthesiologist to monitor bellows and other instruments.
The principles of monitoring oxygen consumption of surgical patients has recently advanced into automated means when intravenous anesthetics are utilized. Such an instrument was disclosed at the IEEE 1975 Regional Six Conference and provides a closed circuit path with automated valving to feed an oxygen mixture into the circuit in accordance with a preset value. Upon exhalation of the oxygen mixture by the patient, the oxygen level is replenished to compensate for the oxygen deficiency indicated by an oxygen sensor in the circuit. A comparator circuit operates a servomotor to feed oxygen into the circuit until a null state is detected between the present reference level and the monitoring oxygen sensor. VO.sub.2 is determined by detecting the amount of oxygen required to re-establish the preset oxygen level.
Although closed system, oxygen consumption monitoring has been realized for applications involving administration or monitoring of oxygen only, the advantages of developing such a system for concurrently applying anesthesia gas and oxygen suggest the greater utility of such a device. In such a system, all venting of dangerous gases is eliminated since the anesthesia gas remains confined to the closed system and patient. The closed system minimizes the amount of anesthesia and oxygen required and facilitates measurement of oxygen consumption and anesthesia uptake with simple and inexpensive feedback circuitry.